Medial coronoid disease is the most common cause of fore limb lameness in young and adult dogs. The term medial coronoid disease includes all the pathological changes which can be attributed to fragmentation of the medial coronoid process. Although fragmentation is an important feature, coronoid disease can be present without any clear fragmentation of bony and cartilaginous structures. In view of this, the typical pathology of the coronoid can be described with the term coronoid dysplasia while using the term medial coronoid disease for the combination of successive secondary pathologies including osteoarthritis. Coronoid dysplasia is a common cause of lameness in the Labrador retriever with a prevalence of MCD of 6% in a screened cohort of Dutch Labrador Retrievers.¹ The diagnosis MCD is based on a combination of clinical evaluation, radiography, and CT imaging. For final diagnosis and treatment, arthroscopy is considered the gold standard.^2,3 Plain radiography following the guidelines of the International Elbow Working Group is used for elbow dysplasia screening programs.¹ The radiographic diagnosis MCD is based on the detection of secondary degenerative joint and bone changes, including periarticular osteophytosis at specific joint locations, ulnar subtrochlear sclerosis (STS), and loss of delineation of the cranial edge of the medial coronoid process, rather than the detection of the primary lesion of the coronoid.

In general, coronoid dysplasia can be present in combination with other types of elbow dysplasia, including osteochondritis dissecans (OCD)-like lesions, incongruity of the elbow joint and ununited anconeal process. Computed tomography (CT) is superior to plain radiography, as it provides assessment of the elbow joint on transverse slices and multiplanar reconstructed images.^3-5 However, neither CT nor radiography are able to assess of the integrity of the subchondral bone and articular cartilage with a high spatial resolution. Arthroscopy allows for precise visual and tactile evaluation of cartilage and subchondral bone. In addition, arthroscopy can be used for surgical intervention in treating the diseased bone and cartilage.

In our study, we evaluated the radiographic, CT, and arthroscopic findings of the elbow joints of Labradors Retrievers diagnosed with MCD. In addition, we compared the data of dogs younger than 12 months of age with dogs older than 12 months of age. A third objective was to assess the correlation of radiographic ulnar STS with the CT ratio between the mean attenuation of the ulnar subtrochlear bone and the mean attenuation of the cortical bone.⁶ The prospective clinical study included 31 Labrador retrievers with MCD. Six healthy Labrador retrievers from a nonrelated study underwent an identical complete radiographic and CT evaluation and served as a control population. Their elbow joints were diagnosed healthy after histological examination.

Ulnar STS (88%) was the most common radiographic findings in age group ≤ 12 months and loss of delineation of the cranial edge of the medial coronoid process (67%) was the most common radiographic findings in age group > 12 months. Fragmentation of the MCP was the most common findings on CT in both age groups, with 94% and 67%, respectively. A displaced fragment (69%) was the most common arthroscopic finding in dogs ≤ 12 months, whereas osteonecrosis and chondromalacia (53%) was the most common pathology in dogs > 12 months. Based on the combination of the primary and secondary lesions, the sensitivity of radiography for detecting MCD in our study was 94% (95% confidence interval, 71.7%–98.9%) in dogs ≤ 12 months and 74% (95% confidence interval, 48.1%–89.1%) in dogs > 12 months. Based on the evidence of fragmentation of the medial coronoid process, the sensitivity of CT in detecting MCD in our study was 94% (95% confidence interval, 71.7%–98.9%) in dogs ≤ 12 months and 67% (95% confidence interval, 41.7%–84.8%) in dogs > 12 months. The sensitivity of the combination of both radiography and CT in detecting MCD was 100% (95% confidence interval, 80.7%–100%) in dogs ≤ 12 months and 80% (95% confidence interval, 54.8%–93.0%) in dogs > 12 months.

Nine dogs from the patient group (n = 9 elbows) diagnosed with MCD and ulnar STS without periarticular osteophytosis were selected and compared with elbow joint data obtained from healthy control group (n = 6 elbows) negative for STS. On reconstructed CT images, ulnar STS could be seen in the intramedullary cavity distal to the ulnar trochlear notch. Radiographic assessed ulnar STS was strongly correlated with CT evaluated ulnar STS.

Although coronoid dysplasia was originally attributed to disturbed endochondral ossification, more resent data also point in the direction of the subchondral bone and a possible relation to STS.^5,7 In a previous study including several other breeds, we assessed dysplastic bone and cartilage of dogs that underwent arthroscopic subtotal coronoidectomy unilateral or bilateral for the treatment of MCD.³ Arthroscopic findings and histopathology of removed bone and cartilage of elbow joints with coronoid dysplasia were compared. The most common arthroscopic finding was fragmentation with softening of the subchondral bone of the central part of the medial coronoid process. In dogs without obvious fragmentation, coronoid dysplasia was characterized by bone softening and chondromalacia. During arthroscopic intervention, dysplastic bone and cartilage was collected for histopathologic assessment. Forty-five slices of formalin-fixed, paraffin-embedded bone and cartilage samples were stained using hematoxylin and eosin (HE) and evaluated. Histopathologic findings primarily showed osteonecrosis of subchondral bone with necrosis within marrow spaces. The articular cartilage showed histopathologic changes characterized by fibrillation, chondrocyte clone formation, and focal cartilage necrosis. The main pathology was found in the subchondral bone and not in the articular cartilage. The osteonecrosis of the coronoid with extension into the bone marrow could be a factor in pathogenesis of STS as found during radiography and CT imaging. The osteonecrosis could also account for the decreased density and irregular structure of the coronoid during CT imaging.

In conclusion, MCD shows a wide range of radiographic and CT abnormalities in the Labrador retriever. Nevertheless, a direct translation of these findings to the arthroscopic evaluation of elbow joint with coronoid dysplasia remains challenging. Part of the radiographic and CT findings could be explained as a result of osteonecrosis and secondary pathology.

References

1.  Lavrijsen IC, Heuven HC, Voorhout G, Meij BP, Theyse LF, Leegwater PA, et al. Phenotypic and genetic evaluation of elbow dysplasia in Dutch Labrador Retrievers, Golden Retrievers, and Bernese Mountain dogs. Vet J. 2012;193(2):486–492.

2.  Fitzpatrick N, Smith TJ, Evans RB, Yeadon R. Radiographic and arthroscopic findings in the elbow joints of 263 dogs with medial coronoid disease. Vet Surg. 2009;38(2):213–223.

3.  Mariee IC, Grone A, Theyse LF. The role of osteonecrosis in canine coronoid dysplasia: arthroscopic and histopathological findings. Vet J. 2014; Epub Apr 18.

4.  Kunst CM, Pease AP, Nelson NC, Habing G, Ballegeer EA. Computed tomographic identification of dysplasia and progression of osteoarthritis in dog elbows previously assigned OFA grades 0 and 1. Vet Radiol Ultrasound; 2014; Epub May 16.

5.  Lau SF, Wolschrijn CF, Hazewinkel HA, Siebelt M, Voorhout G. The early development of medial coronoid disease in growing Labrador retrievers: radiographic, computed tomographic, necropsy and micro-computed tomographic findings. Vet J. 2013;197(3):724–730.

6.  Smith TJ, Fitzpatrick N, Evans RB, Pead MJ. Measurement of ulnar subtrochlear sclerosis using a percentage scale in Labrador retrievers with minimal radiographic signs of periarticular osteophytosis. Vet Surg. 2009 Feb;38(2):199–208.

7.  Lau SF, Hazewinkel HA, Grinwis GC, Wolschrijn CF, Siebelt M, Vernooij JC, et al. Delayed endochondral ossification in early medial coronoid disease (MCD): a morphological and immunohistochemical evaluation in growing Labrador retrievers. Vet J. 2013;197(3):731–738.